Yes, that's currently the question facing the two primary producers of single-aisle aircraft, Airbus and Boeing, and one of them has provided an answer thus far. It's hardly a new idea, however, as a brief review of the history of re-engining of commercial jet aircraft shows.

There are two categories of re-engining to consider: New/different powerplants installed on existing aircraft and changes within ongoing aircraft programs. Both are typically driven by a variety of factors, including improved aircraft performance (such as better takeoff characteristics, or greater range); economic efficiency (reduced cost due to lower specific fuel consumption, decreased maintenance costs due to improved reliability/longevity, etc.); and environmental factors (including noise, and more recently, emissions).

In both cases, there need to be sufficient economic incentives, and/or more stringent regulatory requirements (such as stricter noise regulations) for a change to be considered, whether for existing equipment, or a change to an ongoing program. In the former case, there also will be concerns about the downtime for modifying the aircraft, when it will not be producing revenue, in addition to the direct costs associated with the modification; in the latter, the large-scale cost implications of significant expense will need to be weighed versus factors such as the future competitiveness of the type if it is not re-engined, and the degree of improved performance and operating economics that will be achieved via taking this route.

In the turbine power era, there have been several significant re-engining programs. One of the first involved installing turboprop engines on Convair 240/340/440s that were built as piston-powered aircraft. This involved three different manufacturer's engines, including Napier Eland (Convair 540); the Rolls-Royce Dart (Convair 600/640); and the Allison 501 (Convair 580). This proved to be an excellent way of keeping what proved to be very sturdy airframes in service for much longer than would have been the case had they remained piston-powered.

Early in the jet age, American Airlines and Qantas both re-engined early 707-100 aircraft, to obtain better performance. This involved replacing one type of Pratt & Whitney jet engine with another, going from the JT3C to the JT3D. This sounds like a relatively minor change, but it converted the aircraft from turbojet propulsion to the far more desirable turbofan configuration.

The 707's contemporary, the Douglas (later McDonnell Douglas) DC-8 also had a significant conversion program in its history. Following the advent of more stringent noise regulations in the 1970s, the JT3D now proved to be a liability. Since the DC-8 airframes in the 'stretched' category (-61 and -63) were fairly recent, provided good economics from an airframe standpoint, and had excellent conversion potential as freighters, a number were converted to use CFM56 engines produced by the partnership of GE and Snecma in the 1980s.

On a smaller scale, there was another conversion of cargo aircraft, for a single customer, United Parcel Service. UPS elected to replace Pratt & Whitney JT8Ds with Rolls-Royce Tay engines on a portion of its 727-100 fleet, to meet more stringent noise requirements. In addition, the parcel carrier also made significant upgrades to the cockpits of these aircraft a part of this program.

While it was once common to have only a single engine choice on an aircraft program (although even the early 707s and DC-8s offered both Pratt and Rolls-Royce engines initially), today many programs offer at least two choices, and there have been instances of the so-called "Three on a Wing," where engines from all three major engine manufacturers (GE, Pratt and Rolls) have been available.

On the subject of engine changes within programs, the 737 is the most prominent example, with Boeing launching the now 'Classic' series, beginning with the 737-300, by moving from JT8Ds to a variant of the CFM56 engines utilized in the DC-8 re-engining. Interestingly, even the CFM56 has now been supplanted on a type, when Airbus chose to replace this powerplant, used on the A340-200/300, with Rolls-Royce Trents for the larger -500/600 models. McDonnell-Douglas utilized a higher-bypass version of the JT8D to morph the DC-9 into the MD-80; further development, into the MD-90, involved moving to the IAE V2500 engine.

Now, Airbus has decided to upgrade the A320 program with what it calls the Neo (New Engine Option). The CFM product remains on offer, via the "Leap-X" version of the now-veteran engine; interestingly, Pratt returns to the single-aisle field with its PurePower PW1000G geared turbofan, while IAE's V2500 is dropped.

And thus, it's time to wait and see what Boeing will do with the 737. Will the fourth time be the charm in this case...or will they begin anew, as they appear to be leaning today?

http://airinsight.com/2011/06/27/the-war-of-words/?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+Airinsight+%28AirInsight%29&utm_content=Yahoo!+MailWhat do we know about these engines? The GTF is supposed to run much cooler. CFM says the LEAP will be as cool. But how do you have hotter gas (to get the same low fuel burn as GTF) – do you run more cool air over the metal so it does not burn? CFM says it will use ceramic coatings that have no MRO requirements. If there is cooler air added the efficiency drops and if you use ceramic you are entering something of a new world. Which means risk. Probably easily as much as any risk using a gear.If you find this confusing (how can both be right?) then you are in good company. We understand that every trade journalist who has listened to both sides has come away uncertain. It is a war of words. Both sides have excellent reputations and stories to tell. Both sides are world class engineers – without a doubt the smartest in the field. GE’s engineers produce superb engines. P&W powers anything that flies – it has produced an engine that recently went over Mach 5 and they have a aerospace heritage second to none. Both firms are simply brilliant at what they do.Our take is that the aircraft makers are about to be offered the finest engines, ever. Airlines are likely to see performance yardsticks they have only dreamed about. They can expect the best fuel burn, lowest noise and emissions and amazing time on wing numbers. And because these two firms keep pushing, expect the numbers to creep. P&W can tweak its gear ratio to achieve 25% better fuel burn than current engines with low noise and emissions. CFM will likely add more ceramics to enable it to take current turbofan design to the nth degree.JPRS

--------The inclusion of unique technologies and extensive use of composite materials is intended to achieve an impressive performance - increased propulsive and thermal efficiency allowing for reduced fuel-burn and lower emissions, while allowing the reliability and maintenance costs of the CFM56 to be matched.

LEAP's performance targets are ambitious: 15% improvement in fuel efficiency compared with the best current CFM56 engines, 75% lowering of noise footprint and a 50% reduction in NOx emissions against the Committee on Aviation Environmental Protection CAEP/6 standards.

A number of factors contribute towards these goals. CFM has, for instance, opted for a unique fan blade technology - a 3-D woven composite, using resin transfer moulding - which imparts high durability and a significant weight reduction to the design. The more complex blade shapes this makes possible means fewer actual blades need to be used. There will be just 18 in the LEAP fan - half the number in the earlier CFM56-5B.

The weight benefit continues with the use of composites on the casings too, leaving each LEAP-powered aircraft weighing, according to CFM, 'much less' than a plane using similar engines with conventional metal fan blades and casings.

Aiming at certification in 2015, LEAP has already been selected to power three new single-aisle airliners. When they come into commercial service in 2016, the Airbus A320neo will offer the LEAP-1A as an option, and the -1C will be the sole Western engine for China's Comac C919. One year later, the LEAP-1B is to be the exclusive power-plant aboard Boeing's 737 MAX.

PW1000G - a game-changer

The alternative option on the Airbus A320neo, as well as being the exclusive selection for the Bombardier C-Series, Irkut MS-21 and the Mitsubishi regional jet, Pratt & Whitney's new PW1000G also addresses these same goals of efficiency, emissions and noise - to an essentially identical effect."IPS concepts are featuring in new designs as the need grows across the industry for extra fuel reduction, improved performance and improved maintenance requirements."

It too aims to slash NOx exhaust gases to 50% below CAEP/6, cut noise to a quarter and burn 16% less fuel than today's best engines - with the promise of even greater savings in the future, as new airplanes come into service.

Achieving this, however, has taken the engine's design along a different route.

The first of the 'PurePower' family, the PW1000G arose out of the earlier geared turbofan (GTF) programme, and it is this revolutionary GTF architecture that the company claim make it "game-changing".

Said to be the most efficient in its class, the large, light-weight fan is a 'hybrid-metallic' construction - a high-strength, proprietary design with a black erosion coating and titanium leading edge, which extensive testing has shown offers both the high intrinsic impact-resistance of metals and the low-weight advantages of composites.

Separated from the other engine modules by P&W's innovative gear system, this fan can rotate relatively slowly, while the low-pressure compressor and turbine are able to operate at their higher, optimum speeds, thus increasing engine efficiency and achieving headline reductions in fuel consumption, emissions and noise.

Add to this the NOx-slashing Talon-X combustor, together with deliberate moves towards compact design, fewer parts and inbuilt high-cycle durability and, according to Pratt & Whitney, the era of geared architecture is only beginning.---------